1. Related Application
U.S. patent application entitled "Superconducting Infrared Detector" Ser. No. 373,075 filed on Jun. 29, 1989, now U.S. Pat. No. 5,021,658.
2. Technical Field
The present invention relates to radiant energy detectors; and more particularly, to multispectral superconductive quantum detectors and related method of detection.
3. Background Art
Superconductivity is defined as the absence of electrical resistance and the expulsion of a magnetic field, i.e., the Meissner effect. The superconducting phenomenon occurs at cryogenic temperatures in the neighborhood of 125 Kelvin or less, for example, when using the high temperature, well known superconductors, such as ceramic oxides, which are generally understood to be copper oxides, including but not limited to YBa.sub.2 Cu.sub.3 O.sub.7 La.sub.2-x Ba.sub.x CuO.sub.4, LaSrCuO, BiSrCaCuO, TlBaCaCuO. Also included is BaKBiO and the low temperature elemental and compound superconductors such as Nb, Pb, NbN, Nb.sub.3 S.sub.n, and Nb.sub.3 Ge, for example. Whenever the term superconductor(s), superconductive, superconducting material, or the like is used in this application, it shall mean any material that is capable of becoming superconducting, regardless of the temperature, and regardless of whether such material has yet been identified.
Such superconductive material may take several forms including a thin film. The temperature at which a material actually becomes superconducting is referred to as the superconducting transition or critical temperature (T.sub.c). The amount of superconducting current that a particular superconductor can carry is referred to as the critical current density (J.sub.c).
Superconducting current is composed of and transported by bound pairs of electrons referred to as Cooper pairs. The binding energy between "Cooper" pair electrons is commonly termed the order parameter or the superconducting energy gap. Coherence length is a measure of the distance within which the order parameter changes drastically in a spatially varying magnetic field. The Cooper pair condensate is represented as a wave function with an amplitude, and a phase, reflecting the phase coherence of the Cooper pairs. In a superconductor, this amplitude and phase coherence are maintained over macroscopic distances or coherence lengths. If all the Cooper pairs are broken in a certain portion which extend completely across the width and thickness of the superconductor, the material becomes resistive and exhibits a resistance.
The lack of phase coherence across normal or insulating regions is used to make circuit elements commonly referred to as Josephson Junctions which includes weak links and tunneling junctions. These devices can be assembled into circuits known as superconductor quantum interference devices (SQUID). A weak link may be fabricated by connecting two superconductors with; a thin normal metal, or a constriction made up of a short superconductor section of narrow width, or a superconductor above its transition temperature. A tunneling junction is formed by a tunneling oxide layer, for example. In these examples, the phase of the superconducting current can change across a weak link, since, phase coherence is no longer maintained.
Basically there are two general types of radiant energy detectors; namely, thermal detectors and quantum detectors. A thermal detector, which is sometimes referred to as a bolometer is in effect a very sensitive thermometer whose electrical resistance, for example, varies with temperature; and which is used in the detection and measurement of absorbed thermal radiation energy. A quantum detector changes its electronic characteristics without significant lattice heating in accordance with the radiant flux absorbed by the detector.
Since superconducting materials exhibit a small energy gap, and thus are capable of photoabsorbing long wavelength photons, twenty to thirty microns for example, a great deal of interest has been generated in applying superconductivity to very long wavelength quantum detectors.
Heretofore, superconducting radiant energy detectors have made use of weak links or bolometric structures to detect the intensity of photons or radiant energy. The bolometric devices make use of lattice heating which produces a large change in resistance at the critical temperature. Bolometric detectors are significantly less sensitive (a thousand fold) than quantum (nonequilibrium) detectors, since the noise in bolometric detectors is higher than in quantum detectors.
Conventionally, the typical approach to superconducting quantum detectors is based on the utilization of weak links. However, the formation of weak links in high temperature superconductors in a controlled manner is difficult to achieve. This occurs because the material and processing technology in high temperature superconductors is immature and also because the coherence length in high temperature superconductors is very short. In view of the very short (less than 15 Angstroms) coherence length in high T.sub.c superconductors, the surface conditions at the boundary between the superconductor and the weak link become even more important.
Thus, for example, the tunneling layer thickness needs to be thinner in high temperature superconductors than a corresponding layer in lower temperature superconductors. This combination of the need for very thin tunneling layers and insufficient control of the metallurgical interphase between the superconductor material and the tunneling layer dielectric typically results in unrealizable Josephson Junctions.
Also, detectors based upon weak links are very log in area efficiency. A detector should span as much of the photon receiving pixel area as possible if good quantum efficiency is to be achieved. However, weak links inherently occupy only a very small area relative to a detector pixel size (50.times.50 microns). Hence, many weak links would be required in order to fill such a pixel area. This requirement would impose severe constraints in the manufacture of high quantum yield detectors that are based on Josephson junctions, i.e. weak links or tunneling junctions.
Another difficulty in the utilization of weak links is the expected signal. In achieving a maximum response, the life time of any photo excitation should be maximized. For maximum quasi particle life time, a maximum order parameter is required. The order parameter decreases with increased current, increased magnetic field (H), increase in operating temperature, and interface defects between superconductors and weak link material. High critical temperature superconducting weak link detectors require operating the weak links above or at critical current, which is a condition inconsistent with long quasi particle life time and a maximum order parameter. Thus, the response of such a detector will degrade because of the shorter lifetime expected under such operating conditions. Also, the issue of noise in the resistive state is a large detractor of the weak link approach.